1. Field of the Invention
The present invention relates to a hologram device that performs recording and/or reproduction for a hologram recording medium in which information is recorded using interference fringes between signal light and reference light. Further, the present invention relates to a tilt detection method of detecting a tilt angle as an angle formed by an information recording surface of the hologram recording medium and an optical axis of irradiating light via an objective lens, and a tilt correction method for correction of the tilt angle.
2. Background Art
As disclosed in JP-A-2007-79438, for example, a hologram recording and reproduction method of performing data recording by forming a hologram is known. In the hologram recording and reproduction method, at recording, signal light subjected to spatial light intensity modulation (intensity modulation) according to recording data and reference light provided with a predetermined light intensity pattern are generated and applied to a hologram recording medium, and thereby, a hologram is formed on the recording medium for data recording.
Further, at reproduction, the reference light is applied to the recording medium. In this way, to the hologram formed according to the application of the signal light and the reference light at recording, the same reference light as that at recording (having the same pattern as that at recording) is applied, and thereby, diffracted light according to the recorded signal light component is obtained. That is, a reproduced image (reproduced light) according to the recording data is obtained. The reproduced light obtained in this manner is detected by an image sensor such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor, for example, and thereby, reproduction of the recorded data is performed.
As the hologram recording and reproduction method, a so-called coaxial method of aligning reference light and signal light with the same optical axis and applying them to a hologram recording medium via a common objective lens is known.
FIGS. 14 and 15A and 15B are diagrams for explanation of hologram recording and reproduction using the coaxial method, and FIG. 14 schematically shows a recording method and FIGS. 15A and 15B schematically show a reproduction method.
In FIGS. 14 and 15A and 15B, the case of using a reflective-type hologram recording medium 100 having a reflection film is shown as an example.
First, in the hologram recording and reproduction system, as shown in FIGS. 14 and 15A and 15B, for generation of signal light and reference light at recording and reference light at reproduction, an SLM (special light modulator) 101 is provided. As the SLM 101, an intensity modulator that performs light intensity modulation on incident light with respect to each pixel is provided. As the intensity modulator, for example, a liquid crystal panel may be used.
At recording shown in FIG. 14, signal light provided with an intensity pattern according to recording data and reference light provided with a predetermined intensity pattern are generated by the intensity modulation of the SLM 101. In the coaxial method, the signal light and the reference light are aligned with the same optical axis as shown in the drawing and spatial light modulation on incident light is performed. In this regard, generally, the signal light is aligned at the inside and the reference light is aligned at the outside thereof as shown in the drawing.
The signal light and reference light generated in the SLM 101 are applied to the hologram recording medium 100 via an objective lens 102. Thereby, a hologram reflecting the recording data is formed on the hologram recording medium 100 by interference fringes between the signal light and the reference light. That is, recording of data is performed by the formation of the hologram.
On the other hand, at reproduction, the reference light is generated in the SLM 101 in the manner shown in FIG. 15A (here, the intensity pattern of the reference light is the same as that at recording). Then, the reference light is applied to the hologram recording medium 100 via the objective lens 102.
In response to the application of the reference light to the hologram recording medium 100, as shown in FIG. 15B, diffracted light according to the hologram formed on the hologram recording medium 100 is obtained, and thereby, a reproduced image (reproduced light) with respect to the recorded data is obtained. In this case, the reproduced image is guided as reflected light from the hologram recording medium 100 to an image sensor 103 via the objective lens 102 as illustrated.
The image sensor 103 receives the reproduced image guided in the above described manner with respect to each pixel and obtains electric signals according to amounts of received light with respect to each pixel, and thereby, obtains a detected image with respect to the reproduced image. In this way, the image signals detected in the image sensor 103 are used as readout signals with respect to the recorded data.
Here, the hologram recording and reproduction system as described above has a problem that reproduction performance is deteriorated due to the occurrence of a so-called tilt, as is the case of existing optical disc systems of CD (Compact Disc), DVD (Digital Versatile Disc), or the like.
In the case of the hologram recording and reproduction system, an image shift of the reproduced image occurs due to the occurrence of a tilt and causes deterioration of the reproduction performance.
Further, if a tilt occurs, it may be possible that a shift occurs between the application angle of the reference light at recording of the hologram and the application angle of the reference light applied at reproduction of the hologram, and acquisition of the reproduced image by diffraction is not properly performed. In this respect, deterioration of the reproduction performance is also caused.
Here, for correction of the tilt, first, the detection of the tilt angle (i.e., detection of an error from the tilt angle=0°) is conceivable. That is, correction is performed by cancelling the detected tilt angle.
For the detection of the tilt angle, there is a method using a sensor that directly detects the tilt of the recording medium as a so-called tilt sensor.
Alternatively, a method of performing tilt correction without especially performing the detection of the tilt angle is conceivable.
Specifically, an amount of reproduced light is used as an evaluation index with respect to the tilt angle and correction is performed on the tilt angle at which the largest amount of reproduced light is obtained.